The initiation of herpes simplex virus lytic gene expression proceeds with the activation of immediate early (IE) transcription by VP16 supplied from the input virions. The IE proteins then act to induce and regulate the expression of the remainder of the HSV genome. During latency, viral lytic gene expression does not occur, and the genome persists as an episomal element packaged in chromatin. Reactivation from latency presumably involves activation of the genome in the absence of VP16. One IE protein, ICP0, has been shown to be involved in the process of reactivation from latency in several model systems. ICPO has also been shown to facilitate lytic viral gene expression. While its mechanism of action is unknown, it has been shown to interact with a ubiquitin proteinase. The use of mutants deficient in subsets of the IE proteins provides the means to examine viral gene expression and genome persistence in the absence of lytic gene expression in tissue culture. One mutant, d109, does not express any of the five IE proteins, is completely nontoxic, and establishes a long term relationship with the cell. Gene expression from the persisting genomes is repressed, but can be induced by the addition of ICP0. Therefore, some of the events occurring in d109-infected tissue culture cells are similar to those that may occur with latent genomes in vivo. We propose that the activity of ICP0 results in changes to the cellular pathways involved in repression and derepression of gene expression. Two pathways will be investigated; the histone acetylation pathway and the ubiquitin/proteasome pathway. Data is provided that the action of ICP0 resembles the action of the histone deacetylase inhibitor, trichostatin A, with respect to induction of repressed genomes, effects on cell cycle, and the induction of cellular genes. Unlike other latency model systems, the proposed system is very amenable to quantitative biochemical and molecular characterization. The proposed specific aims are to; i. Examine gene expression and the physical state of persisting viral genomes. ii) Determine how ICP0 alters this state relative to inhibitors of the acetylation and proteasome pathways. iii) Determine the effects of ICP0 on host cell gene expression and histone modification, and compare these to the effects of inhibitors of the acetylation and proteasome pathways. and iv) Determine if changes in the acetylation and proteasome pathways affect HSV infection in a similar way to ICP0. The results of this study will provide insight into how ICP0 affects cells, and how the HSV genome can be repressed and maintained in a latent state, and subsequently reactivated.